The present invention starts out from a drilling or percussion hammer according to the preamble of the main claim. A drilling hammer of this type has been known before (Swiss Patent Specification No. 637 056). In this known drilling hammer, a rotary sleeve is formed integrally with a tool holding fixture and is driven separately, without even participating in the striking process, by a pinion seated on an offset countershaft which is in turn driven by the driving electric motor via the motor shaft which is set off relative to both the rotary shaft and the countershaft. The design of the rotary sleeve which performs the function of a driving shaft for the tool holding fixture, as a hollow sleeve serves the purpose to permit a non-rotary guide tube to be arranged within the said sleeve, at a certain distance and stationary relative thereto, which guide tube encloses in turn an axially reciprocating, likewise non-rotary driving piston. The driving piston acts via an air cushion upon a coaxially seated striker mounted to slide within the same guide tube and imparting its percussion energy directly to the tool in the tool holding fixture.
The reciprocal sliding displacement of the driving piston in the stationary guide tube, which is typical of the design of the striking mechanism, is produced in the known drilling hammer separately by means of a swash plate drive which can be disconnected at desire and which is seated on the intermediate shaft or countershaft and comprises substantially a ring provided with a driving pin which scans, if necessary through the intermediary of a ball bearing, an oblique plane of division of a drum shrunk upon the countershaft, thus translating the rotary movement of the countershaft into the translatory sliding movement of the driving piston which is connected to the other end of the driving pin. The driving pin must be capable of acting upon the driving piston via a ball-type pivoting bearing permitting also axial displacements.
In the case of this known drilling hammer, the rotary energy generated by the electric drive motor is split up, starting from the countershaft, to take two different mechanical routes, i.e. to generate on the one hand the striking movements through the eccentric effect of the swash plate drive, and, on the other hand, to drive the rotary sleeve forming the drive shaft of the tool holding fixture via a separate pinion. The structure of such a drilling hammer is, therefore, extremely complex--it requires a plurality of bearings, sliding faces and pivoting points all of which must be lubricated and are subjected to corresponding wear. The striking mechanism, which is substantially formed by the swash plate drive of the countershaft, acts upon the tool holding fixture, which is driven in the rotary sense, separately and from the outside in offset parallel arrangement, the translatory reciprocating movement being generated externally of the area of the rotary sleeve and the guide tube, only reduced in length by the lever action. The drilling hammers known from the following publications can be classified also to belong to this first category in which the striking action is generated via a swash plate drive: German Disclosure Document No. 32 13 671, German Disclosure Document No. 32 13 672, German Disclosure Document No. 32 05 141, European patent application No. EP 0050 192 A1.
A second basic principle of generating the striking effect with the aid of a stationary, but pivoting knee lever has been known for example from the following publications: German Disclosure Document No. 33 07 482, German Disclosure Document No. 33 05 720. In the case of these knee lever striking mechanisms, the rotary energy supplied by the driving electric motor is also clearly split up on the one hand for generating the striking effect, and on the other hand for ensuring the rotary drive of the drive shaft of the tool holding fixture which is always sleeve-shaped in the broader sense, and this because the parts required for transmitting the externally generated percussion energy have to be arranged coaxially and concentrically with the drive shaft and, preferably, within its hollow interior space. A countershaft or intermediate shaft, which is likewise present in these arrangements and which is driven by the electric motor, primarily via a pinion, carries a collar-like driving part receiving eccentrically the one end portion of the rigid, separately pivoted knee lever. The second arm of the knee lever, which may extend substantially at a right angle relative to the first arm, engages in sliding relationship a slot of a pivot bearing arranged in the driving piston for the striking action. During rotation of the motor shaft and, correspondingly, the countershaft, the first arm is caused to move along a conical surface so that the second arm of the knee lever is forced to perform a pivoting movement whereby the driving piston is caused to perform a reciprocating movement.
The knee point of the knee lever is rigid only as regards the fixed relative angular position of its arms, while it requires a comparatively complex rotary and sliding connection which can be achieved by means of oblong holes and joint pins, because the second arm of the knee lever, which is connected with the driving piston, is in a position only to take over the translatory portion of the eccentric rotary movement of the first arm of the knee lever.
It is not least because of the plurality of the parts that are to be moved, to be seated and to be interconnected via sliding faces, ball bearings, joints and the like, that the known drilling hammers exhibit a very complex design and are, accordingly, expensive to produce and rather difficult to assemble, and in addition very heavy and bulky and, consequently, difficult to handle by the user (nose-heaviness).
Now, it is the object of the present invention to provide a drilling or percussion hammer which can do with a smaller number of moving parts and, accordingly, a smaller number of bearing points, and which offers a particularly simple and compact design while providing the same striking force.